Render statusbar natively on macOS

Instead of trying to mimic native statusbar background rendering in our
own code, use NSWindow setContentBorderThickness:forEdge: to extend the
border that is drawn by the system. This is the canonical way of doing
statusbars ("bottom bar") in AppKit.

The text is still drawn generically, but that's a much smaller issue
than trying to match the border gradient.

include/wx/osx/cocoa/private.h

@@ -329,6 +329,8 @@ public :
 
     virtual void SetRepresentedFilename(const wxString& filename) wxOVERRIDE;
 
+    virtual void SetBottomBorderThickness(int thickness) wxOVERRIDE;
+
     wxNonOwnedWindow*   GetWXPeer() { return m_wxPeer; }
 
     CGWindowLevel   GetWindowLevel() const wxOVERRIDE { return m_macWindowLevel; }